Processes for altering the viscosity of organopolysiloxanes are known. The condensation and equilibration reactions required for this purpose frequently proceed simultaneously.
Phosphonitrilic halides are frequently used as homogeneous catalysts when the formation of cyclic molecules is undesirable. However, these catalysts cannot be removed from the product using physical methods. They are neutralized, and the neutralization products remain in the product.
Heterogeneous catalysts have the advantage that they can easily be removed from the organopolysiloxanes formed. In DE-A44 41 057, for example, acid-activated bleaching earth is used in a heterogeneous mixture as catalyst for continuously preparing organopolysiloxanes by condensation and/or equilibration reactions. In US-A4,792,596, acid-activated montmorillonite is used for this purpose in the form of a fixed-bed catalyst. In the latter process, the catalyst does not have to be removed by specific process steps such as filtration. However, high proportions of from about 10% to 16% of volatile siloxanes, in particular cyclic molecules, are formed in both processes.